1. Field of the Invention
This invention relates generally to telecommunications, and, more particularly, to a method and apparatus for DC feed for a telephone line card.
2. Description of the Related Art
In communications systems, particularly telephony, it is a common practice to transmit signals between a subscriber station and a central switching office via a two-wire bi-directional communication channel. A line card generally connects the subscriber station to the central switching office. A line card typically includes at least one subscriber line interface circuit (SLIC) as well as a subscriber line audio-processing circuit (SLAC). The primary functions of the line card may include supplying talk battery, performing impedance matching, determining whether telephonic equipment is on-hook or off-hook, and handling ringing signals, voice signals, and testing signals.
A telephone system needs DC feed to control the battery feed to a subscriber loop. DC feed delivers enough power for long loop and gradually reduces the power for short loop. A digital signal processing algorithm typically controls the DC feed curve. The DC feed provided to the subscriber loop is usually adjusted in response to an impedance change in the subscriber loop, which, for example, may be caused by off-hook to on-hook transitions, or vice-versa.
In a voltage-feed SLIC, the current on the subscriber line may reach high levels, particularly when the impedance level goes low. The impedance of the subscriber loop may decrease, for example, during a transition from on-hook to off-hook. And, if the subscriber loop is relatively a short loop, a transition from on-hook to off-hook may cause even higher levels on the subscriber line. Unless controlled quickly and expeditiously, these high-level currents may cause the line card to oscillate and perhaps may even damage one or more of the electrical components of the line card.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.